1. Field of the Invention
The invention relates to novel screw elements for multi-shaft screw machines with paired co-directionally rotating and paired fully wiping screw profiles, use of the screw elements in multi-shaft screw machines and a process for the extrusion of viscoelastic compositions.
2. Description of Related Art
Co-directionally rotating twin- or possibly multi-shaft machines, the rotors of which fully wipe one another, have been known already for some considerable time. A comprehensive overview of such screw extruders is provided by the following publication [1]=Kohlgrüber: Der gleichläufige Doppelschneckenextruder [the co-running twin-screw extruder], Hanser Verlag Munich 2007.
In the publication [1], the structure, function and operation of twin- and multi-shaft extruders are particularly explained at length. The screw elements and the way in which they work is devoted a chapter of its own (pages 227-248). Here, the structure and function of conveying, kneading and mixing elements are explained in detail.
Modern screw extruders have a modular system, in which different screw elements can be drawn onto a core shaft. This allows a person skilled in the art to adapt the screw extruder to the respective process task.
When describing screw elements, the cross-sectional profile perpendicular to the axis of rotation—also referred to hereafter as profile for short—is usually considered. A pair of screw elements comprises a screw element with a generating screw profile and a screw element with a generated screw profile.
The regions of a screw profile that are equal to the outer screw radius are referred to as flight land regions. The angle between the starting point and the end point of a flight land region, with respect to the point of rotation of the screw profile, is referred to as the flight land angle. A flight land region that is in contact with the outer screw radius only at one point has the flight land angle 0—the starting point and the end point coincide at a point. The regions of a screw profile that are equal to the core radius are referred to as groove regions. The angle between the starting point and the end point of the groove region, with respect to the point of rotation of the screw profile, is referred to as the groove angle. A groove region that is in contact with the core radius only at one point has the groove angle 0—here, too, the starting point and the end point are identical. The regions of a screw profile that are smaller than the outer screw radius and larger than the core radius are referred to as flank regions. Correspondingly, the angle between the starting point and the end point of a flank region, with respect to the point of rotation of the screw profile, is referred to as the flank angle. The region of a multi-shaft extruder that is penetrated by two barrel bores is referred to as the interstitial region. The two points of intersection of two barrel bores are referred to as the barrel interstice [1].
In polymer preparation and processing, screw machines which are based on the principle of fully wiping profiles have been put to varied use. This is based in particular on the fact that polymer melts adhere to surfaces and, under customary processing temperatures, degrade over time, which is prevented by the self-cleaning effect of the fully wiping screws. Rules for producing fully wiping screw profiles are presented, for example, in publication [1], pages 96-109. Here it is also described that a given screw profile on the first shaft of a twin-screw extruder determines the screw profile on the second shaft of a twin-screw extruder. The screw profile on the first shaft of the twin-screw extruder is therefore referred to as the generating screw profile. The screw profile on the second shaft of the twin-screw extruder follows from the screw profile of the first shaft of the twin-screw extruder and is therefore referred to as the generated screw profile. In the case of a multishaft extruder, the generating screw profile and the generated screw profile are always used alternately on adjacent shafts.
Co-directionally rotating twin- or multi-shaft machines are used in particular for the extrusion of plastic compositions. A plastic composition is understood as meaning a deformable composition. Examples of plastic compositions are polymer melts, in particular thermoplastics and elastomers, mixtures of polymer melts or dispersions of polymer melts with solids, liquids and/or gases.
Extrusion is understood as meaning the treatment of a substance or substance mixture in a co-directionally rotating twin-screw or multi-shaft extruder, as extensively described in [1]. The treatment of substances during an extrusion comprises one or more of the process operations of conveying, melting, dispersing, mixing, degassing and building up pressure.
Extrusion plays a great part particularly in the preparation, compounding and processing of polymers.
In the preparation of polymers, extrusion is performed for example to degas the polymers (see for example [1] pages 191 to 212).
In the compounding of polymers, an extrusion is performed, for example, to mix in additional substances or to mix different polymers, which differ for example in chemical composition, molecular weight or molecular structure (see for example [1] pages 59 to 93). This process referred to as compounding serves for treating the polymer to prepare the finished polymer moulding compound by using the raw polymer materials, which are usually melted, and adding and mixing in fillers and/or reinforcing materials, plasticizers, coupling agents, lubricants, stabilizers, dyes, etc. Compounding often also comprises the removal of volatile constituents, such as for example air and water. The removal of the volatile constituents takes place in this case through openings in the otherwise closed screw barrels, known as the vents. Such vents may expose one or both screw shafts. Since, as is known, extruders convey by friction, at a vent the conveying performance of the extruder is reduced and the degree of filling increases at this point. Compounding may also involve a chemical reaction, such as for example grafting, modification of functional groups or modifications of the molecular weight by deliberately building up or reducing the molecular weight.
When processing polymers, the polymers are preferably brought into the form of a semifinished product, a ready-to-use product or a component. Processing may be performed, for example, by injection moulding, extrusion, film blowing, calendering or spinning. Processing may also comprise mixing polymers with fillers and auxiliary substances and additives as well as chemical modifications, such as for example vulcanization.
On page 73 et seq. in the publication [1], the conveying of the melt and the building up of the pressure are described. The melt conveying zones in extruder screws serve the purpose of transporting the product from one process zone into the next and drawing in fillers. Melt conveying zones are generally partially filled, such as for example when transporting the product from one process zone into the next, when degassing and in holding zones. The energy required for conveying is dissipated and is disadvantageously manifested as an increase in the temperature of the polymer melt. Therefore, screw elements that dissipate as little energy as possible should be used in a conveying zone. For purely conveying melt, thread elements with pitches of approximately once the inside extruder diameter are customary.
A particularly great conveying capacity is required in extruder screws at the points where a second machine that is used for supplying a partial stream of the composition to be extruded is laterally built on. A machine ideally adapted to the requirements at this point would have an increased conveying capacity in comparison with the second shaft on the shaft that has to receive the supplied partial stream. This is not the case, however, with screw profiles according to the prior art.
It is known ([1], page 106) that the conveying capacity of a twin-screw extruder is approximately proportional to the free cross-sectional area. According to the prior art, however, this free cross-sectional area is fixed for each individual element.
Upstream of pressure consumers within the extruder, such as for example backward-conveying elements, mixing elements, backward-conveying or neutral kneading blocks and upstream of pressure consumers outside the extruder, such as for example die plates, extrusion dies and melt filters, there is formed within the extruder a backpressure zone, in which conveying takes place in the fully filled state and in which the pressure for overcoming the pressure consumer must be built up. The pressure build-up zone of an extruder, in which the pressure necessary for discharging the melt is generated, is referred to as the discharge zone. Energy introduced into the polymer melt is divided into useful power for building up pressure and for conveying the melt and into dissipative power, which is disadvantageously manifested as an increase in the temperature of the melt. In the pressure build-up zone, a strong backflow of the melt via the screw flight lands takes place, and as a result an increased input of energy [1]. Therefore, screw elements that dissipate as little energy as possible should be used in a pressure build-up zone.
According to the prior art [1] (see for example page 101), the geometry of the fully wiping screw elements is fixed by specifying the independent variables of the number of flights Z, centreline distance A and outer radius RA. According to the prior art, the flight land angle in the region of which all points of the profile clean the barrel is not a variable that can be set and adapted to the task in question, but is obtained for elements with a flight land region as
      KW    ⁢                  ⁢    0    =            π      Z        -          2      ⁢                          ⁢              arccos        (                  A                      2            ·            RA                          )            where KW0 is the flight land angle of the fully wiping profile in radians and π is the mathematical constant of a circle (π≈3.14159).
According to the prior art [1], the sum of the flight land angles over both elements of a closely meshing pair of elements SKW0 necessarily becomes:
      SKW    ⁢                  ⁢    0    =            2      ⁢      π        ⁢                  -          4      ⁢      Z      ⁢                          ⁢              arccos        (                  A                      2            ·            RA                          )            
Screw profiles may be configured with one or more screw flights. Known screw profiles with just one screw flight are known for good conveying capacity and stiffness during the pressure build-up. They have a very wide screw flight land, which cleans off the screw barrel with a narrow gap. It is known to a person skilled in the art that, on account of the narrow gap, a particularly great amount of energy is dissipated in the melt in the region of the screw flight lands, which leads to instances of strong overheating locally in the product.
This is described, for example, in [1] on pages 160 et seq. for a double-flighted conveying element with the known Erdmenger screw profile. These instances of local overheating can lead to damage occurring in the product, such as for example changing of the odour, colour, chemical composition or molecular weight, or to the formation of inhomogeneities in the product such as gel bodies or specks. In particular, a large flight land angle is detrimental here. Furthermore, in the case of many processes, a high input of energy limits the possible throughput of the twin-screw extruder, and consequently the cost effectiveness.
In co-running twin-screw extruders according to the prior art, therefore, double-flighted screw profiles that have only a narrow screw flight land are predominantly used. However, these are considerably less effective in the pressure build-up than the single-flighted screw profiles.
It is known to a person skilled in the art ([1], pages 129 to 146) that the efficiency in the pressure build-up of double-flighted conveying elements with the known Erdmenger screw profile is approximately 10%. With said efficiency of 10%, a density of the melt of 1000 kg/m3 and a thermal capacity of the melt of 2000 J/kg/K, a rise in pressure of 50 bar leads to a rise in temperature of 25 K ([1], page 120). This heating may lead to damage occurring in the product, such as for example changing of the odour, colour, chemical composition or molecular weight, or to the formation of inhomogeneities in the product such as gel bodies or specks.
Co-directionally rotating twin-screw extruders are established prior art for the processing of thermoplastic polymers. On the other hand, these machines are not yet widely used for the processing of polymers with strongly viscoelastic properties, such as for example rubbers. The viscoelastic behaviour leads to particular phenomena and problems:
The elastic properties have the effect that the products behave in a way similar to solids. Instead of a homogeneous melt, there are soft elastic particles in the partially filled zones of the screw.
These particles or “crumbs” have a low apparent density, as a result of which the volume of the screw flights in open zones of the screw is often not sufficient and the product blocks the openings (for example vents).
The elastic properties bring about a recovery, for example after passing through a gap between the screw and the barrel or in the interstitial region (similar to the “Die Swell” behaviour at a die). This has the effect that some of the particles in the partially filled screw zones are large. Large particles are unfavourable for diffusive processes such as the degassing of volatile components.
The large, recovering particles have the tendency to swell out from the screw flight in open zones of the screw (for example degassing zones) and thereby cause blockages.
The elastic properties make it more difficult to draw the particles into the screw flight or into a gap—the particles tend to move away. As a result, the conveying capacity of the screw is reduced.
The hindered drawing in of particles also reduces the mixing effects and the surface renewal in partially filled zones of the screw, resulting for example in a reduction in the degassing performance in an extruder.
In the case of many commonly used rubbers, the viscoelastic properties are accompanied by a high viscosity, which may lead to a high energy dissipation and consequently to overheating and degradation of the material.
When extruding diene rubbers, such as for example polybutadiene (BR), natural rubber (NR) and synthetic polyisoprene (IR), butyl rubber (IIR), chlorobutyl rubber (CIIR), bromobutyl rubber (BIIR), styrene-butadiene rubber (SBR), polychloroprene (CR), butadiene-acrylonitrile rubber (NBR), partially hydrogenated butadiene-acrylonitrile rubber (HNBR) and ethylene-propylene-diene copolymers (EPDM), an excessively high temperature results in gel formation by cross-linking, which leads to the impairment of mechanical properties of the components produced therefrom. In the case of chloro- and bromobutyl rubber, an elevated temperature may result in the elimination of corrosive gaseous hydrochloric or hydrobromic acid, which in turn catalyzes further decomposition of the polymer.
When extruding rubber compounds which contain vulcanizing agents, such as for example sulphur or peroxides, excessively high temperatures result in premature vulcanization. This results in it no longer being possible to produce any products from these rubber compounds.
Viscoelastic compositions accordingly impose particular requirements on the extruders.
Modern twin-screw extruders have a modular system, in which different known screw elements can be drawn onto a core shaft. This allows a person skilled in the art to adapt the twin-screw extruder to the respective process task. However, the screw elements known from the prior art are mostly not optimally designed for an actual task. Rather, the manufacturers supply screw elements (conveying, kneading and mixing elements) from a fixed modular system irrespective of an actual task.
To be able to process viscoelastic products better, adaptations of the screw fittings are required. With the standard screw elements commonly available on the market, not all process tasks can be satisfactorily accomplished.